How to properly make mock throw an error in Jest?

Question

I’m testing my GraphQL api using Jest.

I’m using a separate test suit for each query/mutation

I have 2 tests (each one in a separate test suit) where I mock one function (namely, Meteor’s callMethod) that is used in mutations.

  it('should throw error if email not found', async () => {
    callMethod
      .mockReturnValue(new Error('User not found [403]'))
      .mockName('callMethod');

    const query = FORGOT_PASSWORD_MUTATION;
    const params = { email: '[email protected]' };

    const result = await simulateQuery({ query, params });

    console.log(result);

    // test logic
    expect(callMethod).toBeCalledWith({}, 'forgotPassword', {
      email: '[email protected]',
    });

    // test resolvers
  });

When I console.log(result) I get

{ data: { forgotPassword: true } }

This behaviour is not what I want because in .mockReturnValue I throw an Error and therefore expect result to have an error object

Before this test, however, another is ran

 it('should throw an error if wrong credentials were provided', async () => {
    callMethod
      .mockReturnValue(new Error('cannot login'))
      .mockName('callMethod');

And it works fine, the error is thrown

I guess the problem is that mock doesn’t get reset after the test finishes.
In my jest.conf.js I have clearMocks: true

Each test suit is in a separate file, and I mock functions before tests like this:

import simulateQuery from '../../../helpers/simulate-query';

import callMethod from '../../../../imports/api/users/functions/auth/helpers/call-accounts-method';

import LOGIN_WITH_PASSWORD_MUTATION from './mutations/login-with-password';

jest.mock(
  '../../../../imports/api/users/functions/auth/helpers/call-accounts-method'
);

describe('loginWithPassword mutation', function() {
...

UPDATE

When I substituted .mockReturnValue with .mockImplementation everything worked out as expected:

callMethod.mockImplementation(() => {
  throw new Error('User not found');
});

But that doesn’t explain why in another test .mockReturnValue works fine…

It looks like your mock is returning an error object, not throwing it. Without seeing your code that you are testing, I can only share the experience I had. I forgot to mock a function called in my mutation, which caused an error to be thrown unintentionally. Perhaps there is something similar happening for you? — May 14, 2018 at 13:54

score 305 · Accepted Answer · 2022-03-24 14:38:02Z

Change .mockReturnValue with .mockImplementation:

    yourMockInstance.mockImplementation(() => {
      throw new Error();
    });

in case you want to assert

   test('the fetch fails with an error', () => {
     return expect(fetchData()).rejects.toMatch('error');
   });

If it’s a promise you can also to .rejects www.jestjs.io/docs/en/asynchronous#resolves–rejects

it throws the error but then the test fails because it has thrown an error. how do we assert? — Mar 7, 2022 at 7:36

score 33 · Accepted Answer · 2021-11-27 03:44:17Z

33

For promises, can use https://jestjs.io/docs/mock-function-api#mockfnmockrejectedvaluevalue

test('async test', async () => {
  const asyncMock = jest.fn().mockRejectedValue(new Error('Async error'));

  await asyncMock(); // throws "Async error"
});

For testing that error was thrown or not, can use https://eloquentcode.com/expect-a-function-to-throw-an-exception-in-jest

const func = () => {
  throw new Error('my error')
}
it('should throw an error', () => {
    expect(func).toThrow()
})

edited Nov 27, 2021 at 3:44

answered Jul 29, 2021 at 18:59

gawkface

1,9342 gold badges26 silver badges29 bronze badges

3

1

You would also need a try and catch in your expect otherwise it would not assert correctly. Can you please improve your answer or reply if I am missing something.

– MG Developer

Nov 24, 2021 at 16:57
1

@MGDeveloper we dont need try-catch while unit testing and using toThrow() (jestjs.io/docs/expect#tothrowerror). If you try that in your tests, it should work. Can also test in here : codesandbox.io/s/jest-playground-forked-euewe?file=/src/… (sandbox content is transient though). I did edit the terminology from “handling” to “testing” if that was confusing

– gawkface

Nov 27, 2021 at 3:42
1

I found the mockRejectedValue helpful in the case that the asynchronous unit I was testing handled the exception thrown in a specific way that I wanted to test, therefore in that case a catch or toThrow() would not be needed.

– CortexCompiler

Aug 10, 2022 at 19:31

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Sean W 5075 silver badges15 bronze badges · Accepted Answer · 2020-10-23 16:06:09Z

12

For Angular + Jest:

import { throwError } from 'rxjs';

yourMockInstance.mockImplementation(() => {
  return throwError(new Error('my error message'));
});

edited Oct 23, 2020 at 16:06

Sean W

5075 silver badges15 bronze badges

answered Apr 2, 2020 at 16:14

AngularBoy

1,5712 gold badges22 silver badges34 bronze badges

3

2

Technically this isn’t a throw in the pure JS sense. You are configuring the mock to return a RXJS observable object which immediately emits an error notification. Still, maybe handy for folks to see here. The accepted answer certainly will make a mock throw an error. In all cases.

– Zach Lysobey

Sep 16, 2020 at 21:57
Only returning throw Error should be enough: yourMockInstance.mockImplementation(() => throwError('my error message'));

– manzapanza

Apr 30, 2021 at 21:42
Actually using mockReturnValue is enough: mockInstance.mockReturnValue(throwError(() => new Error('my error message')))

– RcoderNY

Jun 19, 2022 at 5:41

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How to properly make mock throw an error in Jest?